Over the past many years, a number of different metal based anode materials have been developed for use in the application of impressed current cathodic protection. The metals used either developed their inherent corrosion resistant surface or required the adding of a catalytic surface to facilitate the anode reaction while precluding significant consumption of the substrate metal material.
Of the self protecting metals used as anodes in cathodic protection, the most common were developed by the Duriron Company of Dayton, Ohio more than 30 years ago. These were called Duriron and the later developed Durichlor 51 alloys. Both of these materials are in the cast iron family having iron contents in excess of 75% by weight and a high carbon content of about 0.95%. Their alloy additives include silicon (approx. 14.5% by weight) with small amounts of manganese (0.75%), and chromium (only for the Durichlor 51 alloy at approx. 4.5%). While these materials have performed well for many years, they are heavy, cannot be readily machined or welded, are very brittle and still have unacceptably high anodic corrosion rates for many cathodic protection applications (typically 0.25 pounds per ampere year to more than 1 pound per ampere year depending on the anodic current density and electrolyte surrounding the anode). The only other commonly used ferrous based anode material is the magnetite anode made from a naturally occurring iron ore. This material has natural magnetic properties and is primarily a blend of ferrous and ferric oxides cast in tubular shapes (Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4). Again, the iron content of this anode material is in excess of 75% by weight.
Other self protecting anode metal alloys used in cathodic protection include lead alloyed with either small additions of silver or antimony or a combination of both. Again, the base lead metal is greater than 90% by weight. These metals have worked well as cathodic protection anodes in highly saline environments such as sea water exhibiting consumption rates of a few ounces to a pound or more per ampere of current discharged continuously over a year period (consumption rate is usually expressed in grams, ounces or pounds/ampere year e.g. 1.0 pounds per ampere year). Unfortunately the alloy does not work well in brackish or fresh waters or in most underground environments which precludes its use to provide cathodic protection for structures other than those installed in or very near sea water. Both the high rate of consumption and the possibility of environmental contamination by the lead prevent its use in many otherwise desirable sea water applications.
A different kind of anode material used in cathodic protection utilizes a self passivating valve metal substrate (U.S. Pat. No. 5,062,934, Claim 1) provided with an electro-catalytic surface. The valve metal substrate is usually titanium although aluminum, zirconium, niobium (columbium) and tantalum have also been occasionally used or suggested for such use. The substrate surfaces are coated with either a precious metal or precious metal oxide both of which are typically in the platinum family of metals. Platinum has a low dissolution rate, on the order of a few micrograms per ampere year. The substrate metal serves as the anodic current carrier while virtually all current is transferred between the anode and the surrounding electrolyte only at surfaces where the coating is intact. If the coating is scraped off and the substrate is exposed to the environment, the substrate will passivate or form an oxide film, thus directing the current to flow where the platinum is located. If the substrate did not have this passivating film forming characteristic, the substrate would quickly fail as a result of high faradaic consumption rates (e.g. aluminum has a faradaic consumption rate of 6.0 pounds per ampere year). Use of platinum oxides rather than platinum is popular in the chloroalkali industry.
Unfortunately, the applicable coatings required to produce the capability of anodic current discharge while having extremely low consumption rates include only the platinum metal and metal oxide families, all of which are very expensive and must be applied under expensive and controlled conditions. For the sake of economy, the platinum or platinum oxide is applied thinly. The most commonly used applicable substrate metal is titanium which also bears a relatively high cost of $10-$15 per pound. On the other hand, the substrate material is available in a number of standard shapes and sizes including meshes, rods, tubes, sheets, etc. It is relatively easily machined and welded and can readily be fabricated into a number of shapes.
Some stainless steel has been suggested for use as an anode material. Unfortunately, all such materials tested in the past for their applicable use as cathodic protection anode materials have exhibited unacceptably high consumption (dissolution) rates. The typical cathodic protection electrolytes tested have also suffered from selective pitting and crevice corrosion attack. It is the inventor's observation that this is typically due to oxygen starvation attack of the metal primarily at pits and crevices which naturally and unavoidably occur in the use of these metals as anodes in cathodic protection applications.
It would be desirable to develop a cathodic protection anode comprised of a corrosion resistant ferrous alloy which resists pitting and corrosion while at the same time provides desirable results in an economical fashion.